Semiconductor memory devices may generally be classified as volatile semiconductor devices and non-volatile semiconductor devices. A volatile semiconductor device, for example, a random-access memory (RAM) device, may have a rapid response speed but loses stored data when power is turned off. A non-volatile semiconductor device, such as a read-only memory (ROM) device, may have a slower response speed but maintains the stored data when power is turned off. Conventional ROM devices have the additional limitation that data may not be stored in them after initial programming.
Non-volatile semiconductor devices, such as electrically erasable programmable ROM (EEPROM) devices and/or flash memory devices, are widely used in various electric or electronic devices, because data may be electrically programmed into an EEPROM device and/or a flash memory device, and/or may be erased from the EEPROM or the flash memory device.
In the flash memory device, data may be electrically programmed and/or erased using a Fowler-Nordheim tunneling mechanism and/or a hot electron injection mechanism.
To manufacture a conventional flash memory device, a tunnel oxide layer is typically formed on an active region of a semiconductor substrate, and then a floating gate is formed on the tunnel oxide layer. After a dielectric layer is formed on the floating gate, a control gate is formed on the dielectric layer. Impurities are doped into portions of the active region adjacent to the floating gate so as to form impurity regions.
A conventional flash memory device typically has a stacked structure that includes the tunnel oxide layer, the floating gate, the dielectric layer and the control gate sequentially formed on the semiconductor substrate. When proper voltages are applied to the control gate and the impurity region, electrons are injected into the floating gate from the impurity region so that data is programmed into the conventional flash memory device.
To inject the electrons into the floating gate, the voltages applied to the control gate and the impurity region should be larger than a threshold voltage (Vth) of the conventional flash memory device. The threshold voltage of the flash memory device mainly depends on characteristics of the tunnel oxide layer. When the tunnel oxide layer has poor characteristics, the threshold voltages of unit cells in the flash memory device may be different from one another, which may reduce the reliability of the flash memory device.
To improve the characteristics of the tunnel oxide layer, U.S. Pat. No. 5,591,681 (issued to Dirk J. Wristers, et al.) describes a method of forming a tunnel oxide layer having a surface portion that contains nitrogen, by thermally treating the tunnel oxide layer under a nitric oxide (NO) gas atmosphere. However, since a tunnel oxide layer formed according to the techniques described in the above U.S. patent may not have a dense structure, leakage current through the tunnel oxide layer may be increased.
Korean Laid-Open Patent Publication No. 2004-4797 discloses a method of forming a tunnel oxide layer by nitrifying an oxide layer using a plasma, or by thermally nitrifying the oxide layer. However, the tunnel oxide layer may be damaged during the process of nitrifying of the oxide layer, which may also result in increased leakage current through the tunnel oxide layer.